db Meter is a free sound meter app that measures environmental sound levels and plots them over several hours.

Toggle between four modes to see the maximum, minimum, average and 30-second trail decibel values for different sound measuring applications.

The sound meter's scale is auto-generated to best fit your measured decibel values.

It also features a night mode and dB-calibration by reference sound.

Please note that the maximum decibel reading of most cell phone microphones is limited to approximately 85 dB.
This sound measuring app displays unweighted sound levels.